Drilling for oil and gas produces drill cuttings which are brought to ground surface in the circulating drilling fluid. The drill cuttings are substantially separated from the drilling fluid using various combinations of shale shakers, centrifuges and mud tanks. However, some liquid or moisture remains associated with the solid “cuttings” as a surface layer and, in some cases, internally thereof. The terms “wet cuttings” or “contaminated cuttings” are used interchangeably herein to denote this mixture. In cases where the drilling fluid is hydrocarbon based, the cuttings usually are associated with oil, water and drilling fluid chemical additives.
Disposal of the wet cuttings is often problematic, as the associated liquids are of environmental concern.
This moisture associated with the cuttings also presents problems in handling and treatment. There is a well-known propensity of these cuttings to cake or form unwanted agglomerations when heated and due to mechanical handling and transport operations. This tendency is affected by the amount of liquid present and the nature of the solids and liquids. The tendency may be quite variable.
Current methods for treating wet cuttings generally are not integrated into the drilling operation, but are administered ‘after the fact’. The focus is on how to clean up the mess once drilling is terminated, rather than on how to prevent its occurrence in the first place. With most currently used methods, little, if any, of the liquids are recovered.
On land, the current methods used for wet cuttings disposal are: haul to land-fill; composting; bio-remediation; thermal desorption; and combustion. Off-shore applications usually require shipping the cuttings to shore for processing or deep well injection, as new regulations limit the ability for overboard disposal.
Land fill disposal has long term environmental liability; composting and bio-remediation methods are time consuming and often require mixing with uncontaminated soil prior to final covering; and the known thermal methods do not address concerns with salt and other contaminants.
An additional issue is the loss of drilling fluid. The lost fluid results in increased costs to the drilling operator, as do the increased disposal costs.
Thermal processes are appealing for use in cleaning up cuttings associated with hydrocarbon-base drilling fluid, because they can theoretically achieve a zero residual hydrocarbon level. The thermal desorption processes currently used focus on removal of the liquids after drilling is terminated. The processing units are large and usually involve two stage processes that first remove and then either burn or recover the liquids.
More particularly, the known thermal processes typically involve use of heated screws, rotating kilns or fluidized bed combustion reactors. The equipment used tends to be large scale, fixed capacity units that require a substantially constant feed rate and uniform feed composition. They are not well adapted for handling changes in cuttings generation rates or varying composition while drilling. They are also scale limited due to large capital costs.
The previously described caking and agglomeration tendencies of the cuttings are a significant problem in applying these known thermal processes. When agglomerates or cakes form, the outside initially may be heated and dry out, forming a hard, insulating layer. The inside of the cake remains wet and is difficult to dry due to this insulating effect. It is thus desirable to reduce formation of these cakes or agglomerates in the context of wet cuttings treatment using a thermal process.
Prior art thermal techniques for cleaning drill cuttings are exemplified by the following:
Sample (U.S. Pat. Nos. 4,139,462 and 4,208,285) uses indirect heating of a screw mechanism and jacketed chamber to heat cuttings as they are progressively conveyed through the chamber, venting the gases off. The heating is indirect, via cuttings contact with the screw and vessel walls that are in turn heated by a means such as thermal fluid circulating in jackets that separate the heating medium from the material being heated. Another application using similar conveyance and heating methods is taught by DesOrmeax (U.S. Pat. No. 4,606,283).
McCaskill (U.S. Pat. No. 4,387,514) teaches a process using convection heating with a dry, oxygen rich fixed gas to evaporate liquids from cuttings that are conveyed in a linear, progressive manner from one end of a processor to the other. Vibration is added to prevent agglomeration of the solids on drying. The operating environment is too lean to support auto-ignition of the vapors.
Reed (U.S. Pat. No. 5,570,749) suggests a system that first reduces the amount of liquid on the solids using items such as settling tanks. After reducing the liquid content, the cuttings are routed through an indirectly heated rotating drum unit for final drying.
Daly (U.S. Pat. No. 4,411,074) proposed a rotating kiln process wherein the contaminated cuttings are progressively heated in the rotating drum as they progress through it with the vapors generated being burned.
There are other methods commercially employed for thermal treatment of drill cuttings. One known system is a low temperature process that utilizes heated screws in a heated chamber to evaporate liquids from soils as they are progressively conveyed from one end of the processor to the other. This process uses indirect heat supplied by a hot oil system. The temperatures are 400-500 F. A slight vacuum is maintained to draw gases out of the system.
The Indirect Thermal Desorption Series 6000 System of Newpark Environmental Services is a rotating drum design. This heat-jacketed system has been used to clean drill cuttings.